Not quite true. It also will need remagnetizing after the armature has been removed to install the seal. It still will work afterwards, but with significantly reduced output.
As to the question someone asked (which I must have missed when it was asked, and a quick search didn't turn it up), i.e.
Can a Comet run sweetly with a compact Lucas mag or not?, the answer is 'yes'. However, the mag is 60 years old, so it shouldn't be surprising it needs refurbishing at this point to return it to full health. If I were to rebuild someone's Comet magneto (which I won't do, because I don't do it as a business), I would have no qualms at all about offering a double their money back guarantee against failure for 10,000 miles if no maintenance were performed. If they did just a trivial amount of maintenance at a few points along the way (regrease the cam, measure/adjust the points, inspect and clean carbon from the slip ring) I would increase the length of the warranty. The Lucas magneto is a fundamentally sound design, and is completely capable of providing someone with years of trouble free service after rebuilding before it needs any additional maintenance.
Unfortunately, there are too many magneto rebuilders, some of whom are quite well known and commonly recommended, who use incorrect components and incorrect equipment to remagnetize them (if they remagnetize them at all). I address at length what to look for when searching for someone to correctly rebuild your magneto with proper components at:
http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=464907#Post464907